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The utilization of dissolved organic phosphorus (DOP) by the two toxic dinoflagellates Alexandrium tamarense (Lebour) Balech and Gymnodinium catenatum Grahamm which were isolated from Hiroshima Bay, Japan, was studied. Alexandrium tamarense grew poorly on fructose‐6‐phophate, glucose‐1‐phosphate, glycerophosphate, and ribose‐5‐phosphate with a phosphomonoester bond, although it grew well on the nucleotides adenosine‐5‐diphosphate (ADP) and adenosine triphosphate (ATP), as well as on dissolved inorganic phosphorus (DIP; as metaphosphate, pyrophosphate, tripolyposphate and orthophosphate). The results imply that A. tamarense was able to utilize DOP and DIP from ambient water using nucleotidase, pyrophosphatase and poly‐phosphatase, which hydrolyze phosphodiesters. In contrast, G. catenatum was able to utilize DOP compounds of various molecular weights and structures as well as DIP. In time‐course experiments, alkaline phosphatase activity (APA) was induced at orthophosphate concentrations of 0.43 mmol/L and 3.3 mmol/L for A. tamarense and G. catenatum, respectively, and APA increased with orthophosphate depletion. The experiments also demonstrated that APA was maximum at the optimum temperatures for the growth of A. tamarense and G. catenatum; that is, 15°C and 25°C, respectively. These results suggest that the DIP‐depleted conditions in Hiroshima Bay might have led to the outbreaks of noxious dinoflagellates in recent years.

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